Before dawn on Monday, Mr. Abdeslam, who is French, and Mr. Ayari, a Tunisian, were removed from the high-security prison at Fleury-Mérogis, just south of Paris, and transported by police convoy to Brussels. They will be shuttled to and from Belgium during the trial.
Mr. Abdeslam maintained his intransigence as the trial began, refusing to stand or to confirm his name when asked by the court.
“I was asked to come, and so I came. There is a trial, and I am the actor, and so I came. I was asked to remain silent, and so I remain silent,” he later said.
He went on to criticize the court, and suggested that it was being unduly influenced by what he called “ostentatious” public opinion.
“What I observe is that Muslims are judged, treated in the worst of ways,” he said. “They’re judged mercilessly. There is no presumption of innocence, there is nothing, we’re immediately guilty, voilà.”
Mr. Abdeslam also faces charges for his role in the attacks in France, but the trial in Brussels is centered on the final days of a four-month hunt by the police and security forces. During that time, he eluded arrest, going from house to house across Brussels, before nearly being cornered on March 15, 2016, when several police officers were shot and wounded.
Mr. Abdeslam is accused of being one of the gunmen in that shootout, as is Mr. Ayari, who was hiding with him. As the police advanced on the apartment where they were hiding, Mr. Abdeslam and Mr. Ayari fled, leaving a third man to cover their retreat. That man was killed while Mr. Abdeslam and Mr. Ayari escaped; the defendants were captured three days later.
The trial is being held under high security at the Palais de Justice in central Brussels, one of the biggest courthouses in the world, with a dome that dominates the city’s skyline. The surrounding streets have been blocked off and news reports say that some 200 police officers have been deployed to secure the area. The trial could take weeks or even months.
The judge has questioned the two suspects, and the police officers who were shot are expected to make statements and answer questions. An advocacy group for the victims of the attacks in Brussels and at the country’s largest international airport, in nearby Zaventem, which took place four days after Mr. Abdeslam was taken into custody, is also expected to contribute.
The federal prosecutor will then lay out his case. The trial is a regular criminal trial under Belgian law, held before three judges with no jury.
The proceedings, led by the chief judge, Luc Hennard, started on Monday, when hundreds of people, including lawyers, magistrates, members of the public and journalists queued for hours at freezing temperatures outside the courthouse to get through a single security scanner. The trial nonetheless was able to begin on time.
Another trial, to be held in France, will deal with the attack there and with Mr. Abdeslam’s alleged role, but those proceedings are not expected to start until the end of 2019 at the earliest, according to people close to the case.
The long lead time reflects the monumental work involved in gathering and sorting the evidence, which includes hours of cellphone evidence in multiple languages; the contents of at least one and perhaps more computers; the trail of how the suspects arrived in France; and efforts to pin down their network of forgers, drivers, shelterers and other accomplices.